Photodiode

ABSTRACT

A photodiode that is used in an optical communication system using two different wavelengths, λ 1  and λ 2 (λ 1&lt;λ2 ), and that enables a reduction in the optical crosstalk caused by outgoing light having a longer wavelengths, λ2. A photodiode that receives light having a shorter wavelengths, λ 1 , is provided with an absorption layer made of a material having a bandgap wavelength, λg(λ 1 &lt;λg&lt;λ 2 ), to detect the light having λ 1 . A filter layer that absorbs unwanted light having λ 2  is provided over the absorption layer so that the light having λ 2  cannot return to the absorption layer after passing through it once.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the structure of a photodiodeused in an optical transceiver module for an optical communicationsystem in which one optical fiber is used for carrying out transmissionand reception of signals by using two different wavelengths, λ1 andλ2(λ1<λ2). The photodiode can detect light having a shorter wavelength,λ1, by dexterously eliminating the influence of outgoing light having alonger wavelength, λ2. It should be noted that the photodiode is fordetecting light having a shorter wavelength, not for detecting lighthaving a longer wavelength. To be blocked is the light having a longerwavelength, not the light having a shorter wavelength.

[0003] 2. Description of Related Arts

[0004] When one optical fiber is used for both transmission andreception of signals, a laser diode (LD), which emits outgoing light,and a photodiode (PD), which receives incoming light, are placed usuallyin the same housing or on the same platform. Similarly, when one opticalfiber is used for transmitting signals unidirectionally by using two ormore different wavelengths, two or more PDs are placed usually on thesame platform. A PD is a device having high sensitivity. An LD emitsintense outgoing light to transfer signals to a distant place. Althoughthe PD and LD act at different wavelengths, the PD has sensitivity tothe outgoing light and detects it. When an LD and a PD are placed on thesame terminal, if the PD detects the outgoing light emitted by the LD,this phenomenon is called optical crosstalk. The outgoing light acts asnoise for the PD. When the PD detects the outgoing light, incoming lightcannot be detected accurately. Therefore, it is necessary to minimizethe crosstalk between the PD and LD. Undoubtedly, there can beelectrical crosstalk between a transmitter and a receiver caused by themagnetic coupling between their electric circuits. However, to be solvedhere is the problem of optical crosstalk.

[0005] Researchers and engineers have devised various types oftransceivers that carry out transmission and reception of signals overone optical fiber. Those transceivers employ different methods forseparating the outgoing light and incoming light. The most popularmethod uses an optical wavelength demultiplexer to branch the path forthe outgoing light and the path for the incoming light. Such a method inwhich the two paths are separated spatially can solve the problem ofoptical crosstalk relatively easily. There is a rather specialtransceiver module in which a PD and an LD are arranged in a straightline. Such a method in which almost the same path is used for bothtransmission and reception makes it difficult to solve the problem ofoptical crosstalk.

[0006]FIG. 1 shows a system of simultaneous bidirectional opticalcommunication in which one optical fiber connects a central office and asubscriber for carrying out bidirectional signal transmission by usingtwo different wavelengths, λ1 and λ2. This is a system of wavelengthdivision multiplexing (WDM) bidirectional communication. A centraloffice generates a signal using LD1 and sends it to PD2 at a subscribervia an optical fiber 1, an optical wavelength demultiplexer 2, anoptical fiber 3, an optical wavelength demultiplexer 4, and an opticalfiber 5. The subscriber generates another signal using LD2 and sends itto PD1 at the central office via an optical fiber 6, the opticalwavelength demultiplexer 4, the optical fiber 3, the optical wavelengthdemultiplexer 2 and an optical fiber 7. Thus, signals can be transmittedin opposite directions at the same time over one optical fiber.

[0007] At the central office, the optical wavelength demultiplexer 2 isconnected to the optical fibers 7 and 1 to separate an upstream signaland a downstream signal according to their wavelengths. The outgoinglight carrying downstream signals has the wavelength λ2 and the incominglight carrying upstream signals has the wavelength λ1. The downstreamand upstream signals travel over the one optical fiber 3 at the sametime.

[0008] At the subscriber, the optical wavelength demultiplexer 4 isconnected to the optical fibers 5 and 6 to separate incoming light andoutgoing light. The photodiode PD2 receives incoming light having thewavelength λ2, and LD2 generates outgoing light having the wavelengthλ1. Although not shown, there are individual electric circuits beyondPD2 and LD2.

[0009] The present invention addresses problems related to a transceiverat a central office. Problems at a central office are different fromthose at a subscriber because the aspect related to wavelength isreversed. In the example shown in FIG. 1, the optical wavelengthdemultiplexers 2 and 4 spatially separate the optical paths of theoutgoing light and the incoming light. Therefore, the problem of opticalcrosstalk can be solved by the improvement of the performance of theoptical wavelength demultiplexer, for example. The problem to be solvedby the present invention is the optical crosstalk between the PD and LDat a central office.

[0010]FIG. 2 shows another system in which two signals are transmittedunidirectionally from a central office to a subscriber by usingdifferent wavelengths, λ1 and λ2. This is a system of WDM unidirectionalcommunication. A central office generates two signals by using twodifferent LDs. They are combined at an optical wavelength multiplexer 8and transmitted from the central office to a subscriber over one opticalfiber. At the subscriber, the two signals are separated according totheir wavelengths by an optical wavelength demultiplexer 4. Thephotodiodes PD1 and PD2 selectively receive the signals. Here, crosstalkbetween PD1 and PD2 also poses a problem.

[0011]FIG. 3 shows a typical example of a conventional PD module used asa receiver in an optical communication system in which the optical pathsare separated as shown in FIGS. 1 and 2. This type of PD module is stillmainly used. Lead pins 9 are provided at a circular metal stem 10, atthe center of which a submount 11 supports a PD chip 12. A lens 13 isattached to a cylindrical cap 14 welded to the stem 10 in alignment. Acylindrical sleeve 15 is placed on the cap. A ferrule 16 is insertedinto the mandrel hole of the sleeve 15. The ferrule 16 supports the endof am optical fiber 17. The tip of the ferrule 16 is polished on theskew. The sleeve 15 is covered with a bend limiter 18 to protect theoptical fiber 17. This explains the structure of a PD module currentlyin use. The systems shown in FIGS. 1 and 2 also include LD modules. Inan LD module, only the PD shown in FIG. 3 is replaced by an LD.Therefore, an LD module has a structure similar to that of a PD module,and no explanation about it is provided here. PD modules and LD modulesnow in use employ a metal case, so that optical fibers are arrangedstereoscopically (three-dimensionally). Although high in performance,those modules require centering work at the time of assembly. Thecentering is a time-consuming job and increases the manufacturing cost.Because of the high price, those modules are not suitable in achievingwidespread application.

[0012] Researchers and engineers have been energetically studyingsurface-mounted types for use as less costly PD modules, LD modules, orPD-LD modules. FIG. 4 shows an example of a conventionalsurface-mounted-type module that uses a back-illuminated-type PD. Arectangular silicon platform 19 is provided with a longitudinal,V-shaped groove 20 at the center. The groove is formed by etching. Aslanted mirror face 21 is provided at the end of the V-shaped groove 20.The etching work simultaneously forms the mirror face 21 also. A PD chip23 is fixed directly above the end portion of the V-shaped groove 20.The PD chip 23, a back-illuminated-type PD, is provided with aphoto-sensitive area 24 at the upper zone. The light emerging from anoptical fiber 22 propagates in the V-shaped groove 20 in parallel withthe surface of the silicon platform, is reflected upward by the mirrorface 21, enters the PD 23 at the back side, and reaches thephoto-sensitive area 24. A surface-mounted-type module has no centeringportion. Elimination of centering work accomplishes easy manufacturing.

[0013] Both the PD modules shown in FIGS. 3 and 4 can be used fordetecting the incoming light separated by the optical wavelengthdemultiplexers shown in FIGS. 1 and 2. An optical wavelengthdemultiplexer can be produced, for example, by forming awavelength-selective branching waveguide on a silicon platform. There isalso a prism-type optical wavelength demultiplexer as shown in FIG. 5. Adielectric multilayer film 27 is deposited on the oblique face oftransparent triangular-column glass blocks 25 and 26 for wavelengthselectivity. For example, when light emerges from an optical fiber 28,the light having a specific wavelength is reflected and the other lighthaving a different wavelength is transmitted. In FIG. 5, however, thewavelength selectivity is used for distinguishing the outgoing lightfrom the incoming light. More specifically, the incoming light(wavelength: λ2) emerging from the optical fiber 28 is reflected by themultilayer film 27 and introduced into a PD 30. The outgoing light(wavelength: λ1) emitted from an LD 29 passes through the multilayerfilm 27 and enters the optical fiber 28.

[0014] However, the present invention whose intention is to minimize theoptical crosstalk can be most suitably applied to a transceiver modulein which the optical paths are not separated by an optical wavelengthdemultiplexer. Such a module is called an optical path non-separatedtype in the present invention in order to distinguish from the foregoingoptical path-separated type. In the optical path non-separated type, aPD is placed at the side of the optical fiber and an LD is placed inline with the optical fiber. This type requires no optical wavelengthdemultiplexer. This is advantageous because the size becomes smaller andthe structure becomes simpler. On the other hand, this type has a commonoptical axis for outgoing light and incoming light. As a result, theproblem of optical crosstalk becomes more serious.

[0015]FIG. 6 shows an example of an optical path non-separated type fora module at a subscriber. The wavelengths of the outgoing light andincoming light are opposite to those at a central office. Although notshown, there is a silicon platform in a housing 31. An optical fiber 32is housed longitudinally. An LD 33 is mounted opposite to the end of theoptical fiber 32. A WDM filter 35 is provided at some point in theoptical fiber 32 near its end to carry out wavelength distinction. A PD34 is placed directly above the WDM filter 35. The outgoing light(wavelength: λ1) emitted by the LD 33 is as powerful as 1 mW, forexample. The outgoing light propagates to the outside through theoptical fiber 32. The incoming light (wavelength: λ2) having propagatedthrough the optical fiber 32 from the outside is reflected by the WDMfilter 35, enters the PD 34 at the back side, and is detected by aphoto-sensitive area 36. Whereas the outgoing light is intense, theincoming light is weak. The outgoing light propagates to the WDM filter35 through the same optical fiber 32 in the direction opposite to theincoming light. When passing through the WDM filter, part of theoutgoing light may enter the PD. This intruding light causes the opticalcrosstalk. Notwithstanding the small percentage, the intruding lightbecomes an unignorable noise in comparison with the intensity of theincoming light, because the outgoing light is intense and the incominglight is weak.

[0016]FIG. 7 shows a conventional PD that has a wide range ofsensitivity. When this type of PD is used, the problem of opticalcrosstalk becomes more serious. The structure of the InP-based PD shownin FIG. 7 is based on an epitaxial wafer in which an n-InP buffer layer38, an n-InGaAs absorption layer 39, and an n-InP cap layer 40 arelaminated on an n-InP substrate 37. At the upper zone of the PD, ap-type region 41 and a p electrode passivation layer 44 are formed. Onthe bottom surface, a ring-shaped n electrode 45 and an anti-reflectionlayer 46 are provided. When such a PD is used as a photodetector, thelevel of the noise caused by the outgoing light becomes higher than thesignal level of the incoming light. In other words, the signal/noiseratio (S/N ratio) becomes smaller than one. When an ordinary PD, whichhas sensitivity to both λ2 and λ1, is used, the foregoing undesirablephenomenon occurs.

[0017]FIG. 8 is a graph showing the sensitivity characteristics of thePD shown in FIG. 7. The P portion in the shorter wavelength region, inwhich the sensitivity decreases with decreasing wavelength, correspondsto the bandgap of the InP substrate. The light having a shorterwavelength than that corresponding to the bandgap is not detectedbecause it is absorbed by the InP substrate. The R portion in the longerwavelength region, in which the sensitivity decreases with increasingwavelength, corresponds to the bandgap of the InGaAs absorption layer.The light having a longer wavelength than that corresponding to thebandgap is not detected because its energy is lower than the bandgap ofthe absorption layer. In other words, the PD has sensitivity in a widerange of Q from the bandgap wavelength P of the InP substrate to thebandgap wavelength R of the InGaAs absorption layer. Therefore, the PDhas sufficient sensitivity not only at the 1.3-μm band but also at the1.55-μm band.

[0018] As described above, the PD having a conventional structure asshown in FIG. 7 has sensitivity in a wide range of 1.0 to 1.65 μm asshown in FIG. 8. It is advantageous to have sensitivity in a wide rangeas above because the same PD can be used for both the 1.3-μm band and1.55-μm band. Therefore, the PD having a structure as shown in FIG. 7 ismost widely used for the long-wavelength light employed in opticalcommunications. However, when the PD is used for a transceiver module,the PD also detects the outgoing light in addition to the incominglight, which means that the outgoing light acts as noise. Consequently,the PD is disadvantageous in that optical crosstalk occurs between theoutgoing light and incoming light.

[0019] In the transceiver module shown in FIG. 6, not all the intenseoutgoing light (wavelength: λ1) emitted from the LD placed on thesilicon platform (silicon bench) enters the optical fiber. The lightemitted from the LD spreads out at a considerably wide angle. Some ofthe light strikes the silicon platform and plastics to be scattered. Thesilicon platform is transparent to the outgoing light. The outgoinglight having entered the space made by the silicon platform andtransparent plastics passes through the silicon, is reflected, and isscattered. Various complicated scattered rays of light are producedaccording to the distribution of the plastics, the shape of the siliconplatform, and the arrangement of the other devices. When looked from thePD, the entire silicon platform shines brightly due to the scattering ofthe outgoing light. Such components of the outgoing light that enter thePD through various paths other than the designed path are called“scattered light” or “stray light.”

[0020] Some components of the outgoing light enter the PD from variousdirections and at various heights. They enter the PD at the back side,at the front side, and at the side face. Such components of the outgoinglight that enter the PD without entering the optical fiber cause thecrosstalk. Such crosstalk caused by the scattered light (stray light)that does not pass through the WDM filter cannot be suppressed by theimprovement of the performance of the WDM filter. When the output powerof the LD is increased, the outgoing light propagating through theoptical fiber increases the amount of the leakage at the WDM filter. Thecomponent of the outgoing light emitted from the LD that enters the PDafter being refracted and reflected at the WDM filter is called “leakagelight.”

[0021] The unexamined Japanese patent publication (Tokukaihei)No.4-213876 entitled “Photodetector” proposes a photodetector that is aPD comprising two stages of absorption layers. A layer structure thatabsorbs 1.55-μm light is provided on an InP substrate and a p electrodeis provided on the layer structure. On part of the layer structure,another layer structure that absorbs 1.3-μm light is provided andanother p electrode is provided on this layer structure. A common nelectrode is provided on the bottom surface of the InP substrate.Consequently, the photodetector has a two-stage structure in which PD1for absorbing the light having λ1(1.3 μm) is placed at the top and PD2for absorbing the light having λ2(1.55 μm) is placed at the bottom.

[0022] The light having λ2 and the light having λ1 enter thephotodetector at the front side. Since the light having λ2 has a longerwavelength, it passes through the upper layer structure and reaches thelower layer structure to generate optical current there. In other words,PD2 absorbs the light having λ2 at the bottom. The light having λ1,which is shorter, is absorbed by PD1 in the upper structure to generateoptical current there. In other words, PD1 can detect the light havingλ1 at the top. In order to prevent the penetration of the light havingλ1 into the lower structure, a layer having a thickness of d=mλ1/(2n),where m is a plus integer and n is a refractive index, is providedbetween PD1 and PD2. The object of this layer is to reflect the lighthaving λ1 upward so that the light having λ1 cannot enter PD2. Hence,this layer is called a “selective reflection layer.” If the light havingλ1 enters PD2, the light causes PD2 to generate optical current, so thatcrosstalk occurs. The selective reflection layer is provided to preventthis type of crosstalk.

[0023] However, this patent application provides no preventive measureagainst crosstalk in the opposite case. Such a case is out of itsexpectations. There is no measure against the phenomenon that the lighthaving λ2 is reflected by the n electrode at the bottom, returns to PD1,and adversely affects its performance. Since the selective reflectionlayer provided between PD1 and PD2 reflects the light having λ1 buttransmits the light having λ2, the light having λ2 reflected at thebottom face can pass through the layer upward.

[0024] Another unexamined Japanese patent publication, (Tokukaihei)No.9-166717, entitled “Optical receiver module and optical transceivermodule” proposes a photodetector for a system in which two signalshaving different wavelengths, λ1=1.3 μm and λ2=1.55 μm, are transmittedthrough one optical fiber. A first photodiode, PD1, absorbs the lighthaving λ1 and transmits the light having λ2. A second photodiode, PD2,placed behind PD1, absorbs the light having λ2. Two independent PDs arecombined in tandem. They are not such composite devices as describedabove. The photodiode PD1 has an absorption layer that has anintermediate bandgap wavelength as expressed in λ1<λg<λ2, where λgrepresents the bandgap wavelength of the absorption layer. Since λg islonger than λ1, the absorption layer absorbs and detects the lighthaving λ1. The light having λ2 passes through PD1 and is detected byPD2. FIG. 9 shows the structure of the PD for absorbing the light havingλ1 proposed by Tokukaihei No.9-166717. The PD is placed in anintermediate place to transmit the light having the longer wavelength.For this purpose, the PD has another opening at the side opposite to thelight-entering face to allow the light having the longer wavelength toleave the PD. The PD can be called a dual opening type, because it hasopenings at both sides for transmitting light.

[0025] An n-InP buffer layer 51, an n-InGaAsP absorption layer 52(λg=1.42 μm), and an n-InGaAsP window layer 53 are grown epitaxially onan n-InP substrate 50. At the center portion, Zn is diffused to providea p-type region 54. The center portion of the p-type region 54 iscovered by an anti-reflection layer 56. Around the anti-reflection layer56, a ring-shaped p electrode 55 is provided. At the outside of the pelectrode 55, a passivation layer 57 is formed to protect the edgeportion of the pn junction. A ring-shaped n electrode 58 is formed onthe bottom surface of the InP substrate 50. The inside of the nelectrode 58 forms an opening and is covered by an anti-reflection layer59. Both the front and back sides have openings for transmitting light.The ring-shaped electrodes are provided without overlapping with theseopenings. The anti-reflection layers are provided at the openings toprevent incident light from attenuating due to reflection.

[0026]FIG. 10 shows a transmittance spectrum of the InGaAsP absorptionlayer 52 (λg=1.42 μm). The mixing ratio of its quaternary mixed crystalis decided for the bandgap wavelength to take an intermediate valuebetween 1.3 μm and 1.55 μm. The measured result proves the designconcept. The light having a wavelength shorter than 1.4 μm is absorbedalmost completely, which means that the light practically does not passthrough the layer. A wavelength of 1.42 μm forms the boundary condition.Almost one hundred percent of the light having a wavelength longer than1.5 μm passes through the absorption layer. The transmittance varieswith the thickness. The absorption layer has an enough thickness so thatthe light having a shorter wavelength can be absorbed completely.

[0027] The present invention intends to prevent a PD that detects thelight having a shorter wavelength from suffering the crosstalk caused bythe light having a longer wavelength. In the explanation below, theshorter wavelength, λ1, is supposed to be 1.3 μm and the longerwavelength, λ2, to be 1.55 μm in order to specifically show the relationbetween the two wavelengths. In the present invention, however, λ1 is inthe range of 1.2 to 1.38 μm, and λ2 is in the range of 1.45 to 1.65 μm.At a central office, if a PD as shown in FIG. 7, which usually hassensitivity in a wide range, is used as a photodetector, it also detectsthe scattered light and leakage light of the outgoing light. At acentral office, the incoming light has a wavelength of 1.3 μm, and theoutgoing light, 1.55 μm. This combination of wavelengths is advantageousin eliminating the effect of the outgoing light. Dexterous exploitationof the basic properties of the semiconductor enables the production of aPD for a central office that detects the incoming light (λ1=1.3 μm) butdoes not detect the outgoing light (λ2=1.55 μm). This can beaccomplished by selecting the bandgap wavelength λg of the absorptionlayer of the PD to satisfy the following formula:

λ1(incoming light)<λg<λ2(outgoing light).

[0028] This is possible because the two wavelengths at a central officehave such an advantageous relationship. If the bandgap wavelength λg ofthe absorption layer is decided to be 1.35 to 1.45 μm, for example, thenthe absorption layer should have a desirable quality that it detects theincoming light but does not detect the outgoing light. A bandgapwavelength can be adjusted to 1.35 to 1.45 μm by using a quaternarymixed crystal of InGaAsP. In the present invention, however, λg is inthe range of 1.3 to 1.5 μm.

[0029]FIG. 10 shows a light transmittance of an InGaAsP quaternarymixed-crystal layer having a bandgap wavelength of 1.42 μm. Theabsorption layer of the PD shown in FIG. 9 is made of such amixed-crystal. Its transmittance is zero for 1.3-μm light (incominglight). In other words, it absorbs and detects 1.3-μm light completely.On the other hand, its transmittance is almost one hundred percent for1.55-μm light (outgoing light). In other words, it transmits 1.55-μmlight almost completely, which means it does not detect 1.55-μm light.Therefore, a PD as shown in FIG. 9, which has wavelength selectivity,can be used singly as a photodetector at a central office. The PD shownin FIG. 9 has an opening both at the front and back sides, becausebehind the PD another PD for detecting 1.55-μm light is to be placed.However, when a PD is used at a central office, only one opening isrequired because the PD has only to absorb 1.3-μm light. If the PD is aback-illuminated type, the front side is covered by the p electrode. Ifthe PD is a front-illuminated type, the back side is covered entirely bythe n electrode. Such a PD can be used as a photodetector at a centraloffice without modification.

[0030]FIG. 11 shows a back-illuminated type PD conceived on the basis ofthe above-described consideration for the use in a central office.Although the PD is almost the same as that shown in FIG. 9, it has aslightly different structure in the vicinity of the p electrode. Ann-InP buffer layer 61, an n-InGaAsP absorption layer 62 (λg=1.42 μm),and an n-InP cap layer 63 are grown epitaxially on an n-InP substrate60. At the center portion of the chip, Zn is diffused to provide ap-type region 64. A p electrode 65 having no opening is provided tocover almost the entire p-type region 64. Since the front side is notrequired to admit light, no opening is provided there. At the outside ofthe p electrode 65, a passivation layer 67 is formed to protect the edgeof the pn junction. Since light enters the PD at the back side, theback-side structure is the same as in FIG. 9. A ring-shaped n electrode68 is formed on the bottom surface of the InP substrate 60. The insideof the n electrode 68 forms an opening for admitting light and iscovered by an anti-reflection layer 69. The ring-shaped electrode isprovided without overlapping with the opening.

[0031] It should be possible to use a PD having such a structure as aphotodetector at a central office. Nevertheless, the present inventorsfound that when such a PD is used as a photodetector at a centraloffice, crosstalk occurs due to the influence of 1.55-μm light (outgoinglight at the central office). It was out of the present inventors'expectations. The InGaAsP absorption layer 62 has a bandgap wavelengthof 1.42 μm. Since it is shorter than 1.55 μm, the present inventorsexpected the absorption layer to be insensitive to 1.55-μm light as anideal case. However, the result showed differently. The presentinventors found that when the absorption layer 62 has a thickness of 5μm, it detects about 0.2% of 1.55-μm light. The absorption layer absorbs100% of 1.3-μm light. The fact that the absorption layer detects 1.55-μmlight even in small magnitudes poses a problem. Although 1.55-μm lighthas an energy lower than the bandgap energy, there are some impuritylevels in the bandgap, and these levels effect the slight sensitivity to1.55-μm light. At a central office, there is imbalance in intensity oflight. Whereas the 1.55-μm outgoing light generated in the office isintense, the 1.3-μm incoming light having propagated over an opticalfiber is weak. The 1.55-μm outgoing light is more intense than the1.3-μm incoming light by orders of magnitude. Therefore, even the 0.2%sensitivity can produce an un-ignorable magnitude in noise level becausethe multiplier has a considerable magnitude.

SUMMARY OF THE INVENTION

[0032] An object of the present invention is to offer a photodiode inwhich crosstalk caused by the intrusion of intense 1.55-μm outgoinglight into the 1.3-μm light detection portion at a central office can bereduced. This reduction in crosstalk can be accomplished by devising theconfiguration of the photodiode.

[0033] The present invention intends to reduce the crosstalk caused by1.55-μm light by preventing or impeding the return of the outgoing light(λ2=1.55 μm) to the absorption layer after passing through theabsorption layer once. In order to achieve this purpose, a layer forabsorbing 1.55-μm light is additionally provided at the inside or at theoutside of a PD. In the present invention, this additionally providedabsorption layer is called a “filter layer.” Since these filter layersabsorb unwanted 1.55-μm light, the light does not return to theabsorption layer for 1.3-μm light, or its intensity is notably reducedeven if it returns. This measure can effectively reduce the crosstalk tothe 1.3-μm light by the 1.55-μm light predominant at a central office.The methods for providing filter layer for absorbing the 1.55-μm lightinclude the following four types:

[0034] Type 1: To replace the InP cap layer by a thick InGaAs cap layer;

[0035] Type 2: To provide an InGaAs filter layer by epitaxial growth andremove its peripheral region;

[0036] Type 3: To laminate an InGaAs filter layer on the p-type regionby the selective growing method; and

[0037] Type 4: To laminate a filter layer made of a plastic resin oranother material on the entire top surface of a chip.

[0038] When a material that absorbs 1.55-μm light is provided on thep-type region as mentioned above, the 1.55-μm light (outgoing light)that has once passed through the p-type region does not return to theabsorption layer, or it loses its intensity notably even if it returns.The methods are not limited to the above-mentioned four types providingthat a newly conceived method can achieve a similar effect.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In the drawings:

[0040]FIG. 1 is a schematic diagram showing a system of simultaneousbidirectional optical communication in which one optical fiber connectsa central office and a subscriber for carrying out bidirectional signaltransmission by using two different wavelength, λ1 and λ2;

[0041]FIG. 2 is a schematic diagram showing a system of simultaneousunidirectional optical communication in which one optical fiber connectsa central office and a subscriber for carrying out unidirectional signaltransmission by using two different wavelength, λ1 and λ2;

[0042]FIG. 3 is a partial cutaway and partial cross-sectional view of aconventional PD that is housed in a metal housing and that has athree-dimensional structure;

[0043]FIG. 4 is a cross-sectional view of a conventionalsurface-mounted-type PD module;

[0044]FIG. 5 is a schematic diagram showing the constitution of aprism-type optical wavelength demultiplexer comprising glass blocksprovided with a dielectric multilayer film;

[0045]FIG. 6 is a schematic diagram showing the constitution of aconventional optical transceiver module at a subscriber that has an LDfor generating outgoing light (wavelength: λ1) and a PD for receivingincoming light (wavelength: λ2) (λ1<λ2);

[0046]FIG. 7 is a cross-sectional view of a conventional PD having anInGaAs absorption layer having sensitivity in a wide range including λ1and λ2;

[0047]FIG. 8 is a graph showing the sensitivity-wavelengthcharacteristic of a conventional PD, in which the axis of abscissarepresents wavelength (μm) and the axis of ordinate representssensitivity (A/W);

[0048]FIG. 9 is a cross-sectional view of the 1.3-μm-light-detecting PDin the combination of a 1.3-μm-light-detecting PD and a1.55-μm-light-detecting PD that is disclosed in the published Japanesepatent application Tokukaihei 9-166717;

[0049]FIG. 10 is a graph showing the light transmittance-wavelengthcharacteristic of the InGaAsP layer of the PD shown in FIG. 9, in whichthe axis of abscissa represents wavelength (μm) and the axis of ordinaterepresents transmittance (%);

[0050]FIG. 11 is a cross-sectional view of a 1.3-μm-light-detecting PDproduced by closing the front-side opening of the 1.3-μm-light-detectingPD shown in FIG. 9 by the p electrode;

[0051]FIG. 12 is a cross-sectional view of a PD for detecting lighthaving λ1(λ1<λ2) of Type 1 of the present invention, in which an InGaAscap layer that can absorb light having λ2 is provided on the entireabsorption layer;

[0052]FIG. 13 is a cross-sectional view of a PD for detecting lighthaving λ1(λ1<λ2) of Type 2 of the present invention, in which an InGaAsfilter layer that can absorb light having λ2 is provided only on thecenter portion of the cap layer;

[0053]FIG. 14 is a cross-sectional view of a PD for detecting lighthaving λ1(λ1<λ2) of Type 3 of the present invention, in which an InGaAsfilter layer that can absorb light having λ2 is selectively grown at theinside of the p electrode.

[0054]FIG. 15 is a cross-sectional view of a PD for detecting lighthaving λ1(λ1<λ2) of Type 4 of the present invention, in which a plasticresin that can absorb light having λ2 is applied to the entire topsurface of the chip.

[0055]FIG. 16 is a cross-sectional view of an example in which the Type4 PD of the present invention is used for a surface-mounted-type PDmodule at a central office.

[0056]FIG. 17 is a cross-sectional view of an example in which the Type4 PD of the present invention is used for a surface-mounted-typetransceiver module at a central office.

DETAILED DESCRIPTION OF THE INVENTION

[0057] The present invention features a back-illuminated-type PD thathas on the p-type region a layer made of a material that absorbs lighthaving λ2 for preventing the return of the light after reflection sothat the light cannot pass through the absorption layer twice. Althoughthis PD does not prevent the first passage of light having λ2 throughthe absorption layer, it can prevent the second passage. This concept isrealized through the process described below.

[0058] The PD for 1.3-μm light shown in FIG. 9 detects 0.1% of 1.55-μmlight. However, the PD for 1.3-μm light produced for central-office useshown in FIG. 11 has a higher sensitivity to 1.55-μm light; it detects0.2% of 1.55-μm light. Although the two PDs have the sameabsorption-layer thickness, 5 μm, the effects differ by a factor of two.The study on the difference revealed that the sensitivity increase iscaused by the phenomenon that the 1.55-μm light is reflected by the pelectrode 65, which covers the entire p-type region, and passes throughthe absorption layer once more. In FIG. 11, the thicker arrow representsthe light (λ1=1.3 μm) to be detected. This light is absorbed by theInGaAsP absorption layer 62, generates an optical current in proportionto its intensity, and disappears. This function is in accordance withthe design concept and poses no problem. At a central office, however,when the intense outgoing 1.55-μm light enters at the back side, itpasses through the InGaAsP absorption layer 62 upward from under,generating a slight optical current. This does not terminate the actionof the light. It is reflected by the under side of the p electrode 65and passes through the absorption layer 62 once more in the oppositedirection, generating optical current again. As a result, crosstalkoccurs twice. This is the reason why the PD shown in FIG. 9 has asensitivity of 0.1% to 1.55-μm light and the PD shown in FIG. 11 has asensitivity two times as much, 0.2%.

[0059] The present invention intends to reduce the crosstalk caused by1.55-μm light by preventing or impeding the return of the 1.55-μm lightto the absorption layer after passing through the absorption layer once.In order to achieve this purpose, a layer (filter layer) for absorbing1.55-μm light is additionally provided at the inside or outside of thePD structure. Since these filter layers absorb unwanted 1.55-μm light,the light does not return to the absorption layer for 1.3-μm light, orits strength is notably reduced even if it returns. This measure caneffectively reduce the crosstalk to the 1.3-μm light by the 1.55-μmlight predominant at a central office. The present invention provides amaterial that absorbs light having λ2 on the p-type region. Theproviding methods are classified into four groups, which are explainedbelow, according to their differences.

[0060] Type 1: Thick Cap Layer (FIG. 12)

[0061]FIG. 12 shows an embodiment of the present invention in which athick InGaAs cap layer (filter layer) is provided on the absorptionlayer. An n-InP buffer layer 61, an n-InGaAsP absorption layer 62(λg=1.42 μm), and an n-InGaAs cap layer (filter layer) 78 are grownepitaxially on an n-InP substrate 60. At the center portion of the chip,Zn is diffused to provide a p-type region 64. Another p-type region isformed at the periphery of the chip at the same time. This additionalp-type region is referred to as a Zn-diffused shield layer 71. Aring-shaped p electrode 65 is provided on the p-type region 64. Directlyunder the ring-shaped p electrode 65, there lies a portion in which then-InGaAs cap layer 78 is converted into the p-type region. Since the pelectrode has an opening at its inside, it is called a ring-shapedelectrode. The p electrode 65 can have any inside and outside shapes,such as a circle, oval, square, pentagon, or hexagon (this is to beapplied also to Types 2 to 4).

[0062] An InP layer is used usually as the cap layer on the absorptionlayer. However, InP cannot absorb light having λ2 because it has a widebandgap. Consequently, Type 1 employs InGaAs in place of InP as the caplayer so that the layer itself can absorb light having λ2. BecauseInGaAs has a narrow bandgap, it can absorb light having λ2. Therefore,the cap layer can also be called a filter layer.

[0063] An anti-reflection layer 72 is formed on the p-type region 64 atthe inside of the p electrode 65. Since the PD is a back-illuminatedtype, the anti-reflection layer 72 is not for admitting light, but forallowing the unwanted light having λ2 to leave the PD without beingreflected. At the outside of the p electrode 65, a passivation layer 67is formed to protect the edge of a pn junction 66. Since light entersthe PD at the back side, the back-side structure is the same as in FIG.9. A ring-shaped n electrode 68 is formed on the bottom surface of theInp substrate 60. The inside of the n electrode 68 forms an opening andis covered by an anti-reflection layer 69. As with the p electrode 65,the n electrode 68 can have any inside and outside shapes, such as acircle, oval, square, or octagon (this is to be applied also to Types 2to 4). Table I shows the thickness and carrier concentration of theepitaxial layers. TABLE I Carrier Layer's thickness concentrationLayer's name (μm) (cm⁻³) InGaAs cap layer (filter layer) (78) 5 p = 10¹⁸InGaAsP absorption layer (62) 5 n = 10¹⁵ InP buffer layer (61) 4 n =10¹⁵ InP substrate (60) 200  n = 10¹⁸

[0064] The p-InGaAs cap layer is produced by converting the n-InGaAs caplayer 78 into a p type by the diffusion of Zn. The layer is as thick as5 μm in order to absorb and attenuate light having λ2. The unwantedlight having λ2 passes through the absorption layer upward from under.It is absorbed by the cap layer 78, and the remaining light leaves thePD. Dissimilar to the PD in FIG. 11, the light having λ2 is notreflected by the p electrode, so that it does not return to theabsorption layer. The anti-reflection layer 72 provided at the topsurface is for preventing the reflection of the light having λ2=1.55 μmwhen it leaves the PD, not for preventing the reflection of lightincident from outside. When stray light having λ2 enters the PD fromabove, the InGaAs cap layer can prevent the influence of the light.Because the layer has high hole concentration, the hole-electron pairsproduced by the absorption of the light having λ2 recombine withoutgenerating optical current. In short, Type 1 replaces the InP windowlayer in the internal structure of a conventional PD by a thick InGaAscap layer for the purpose of absorbing light having λ2 so that it cannotreturn to the absorption layer.

[0065] Type 2: Local and Thick Filter Layer (FIG. 13)

[0066]FIG. 13 shows another embodiment of the present invention in whicha thick InGaAs filter layer is provided only at the center portion of achip. Whereas Type 1 shown in FIG. 12 has a uniformly thick InGaAsP caplayer, Type 2 is designed with the concept that the light having λ2passing through the center portion of the chip can be handled with athick filter layer provided only at the center portion. Consequently,the cap layer is made of InP.

[0067] An n-InP buffer layer 61, an n-InGaAsP absorption layer 62(λg=1.42 μm), an n-InP cap layer 63, and an InGaAs filter layer 74 aregrown epitaxially on an n-InP substrate 60. At the center portion of thechip, Zn is diffused to provide a p-type region 64. Another p-typeregion is formed at the periphery of the chip at the same time. Thisadditional p-type region is a Zn-diffused shield layer. The peripheralportion of the InGaAs filter layer 74 is removed so that the centerportion, which is converted into a p-type region, can be remained. Aring-shaped p electrode 70 is provided on the protruding portion at thecenter. An anti-reflection layer 72 is formed on the InGaAs filter layer74 at the inside of the p electrode 70. Since the PD is aback-illuminated type, the anti-reflection layer 72 is not for admittinglight, but for allowing the unwanted light having λ2 to leave the PDwithout being reflected.

[0068] Directly under the ring-shaped p electrode 70, there lies thep-InGaAs filter layer 74, followed by the p-InP cap layer and thep-InGaAsP absorption layer. Usually, an InP cap layer is placed on theabsorption layer. However, InP has a wide bandgap and cannot absorblight having λ2. Consequently, Type 2 further laminates an InGaAs layer,which can absorb light having λ2, on the InP cap layer. Dissimilar toType 1, this type absorbs light having λ2 by the thick InGaAs filterlayer provided only at the center portion.

[0069] At the outside of the InGaAs filter layer 74, a passivation layer67 is provided to protect the edge of the pn junction. Since lightenters the PD at the back side, the back-side structure is the same asin FIG. 12. A ring-shaped n electrode 68 is formed on the bottom surfaceof the n-InP substrate 60. The inside of the n electrode 68 forms anopening and is covered by an anti-reflection layer 69. The p-InGaAsfilter layer 74 has a thickness of 5 μm and a carrier concentration ofp=10¹⁸cm⁻³. The layer is as thick as 5 μm in order to absorb andattenuate light having λ2. The unwanted light having λ2 passes throughthe absorption layer upward from under. It is absorbed by the filterlayer, and the remaining light leaves the PD. Dissimilar to the PD inFIG. 11, the light having λ2 is not reflected by the p electrode, sothat it does not return to the absorption layer.

[0070] The anti-reflection layer 72 provided at the top surface is forpreventing the reflection of the light having λ2=1.55 μm when it leavesthe PD, not for preventing the reflection of light incident fromoutside. When stray light having λ2 enters the PD from above, the InGaAsfilter layer 74 can prevent the influence of the light. Because thelayer has high hole concentration, the hole-electron pairs produced bythe absorption of the light having λ2 recombine without generatingoptical current. In short, Type 2 additionally provides a thick InGaAsfilter layer for absorbing light having λ2 so that it cannot return tothe absorption layer.

[0071] Type 3: Selectively Grown Thick Filter Layer (FIG. 14)

[0072]FIG. 14 shows yet another embodiment of the present invention inwhich a thick filter layer is provided at the center portion of a chip.In Type 2 shown in FIG. 13, the layers up to and including the filterlayer are formed by the epitaxial growth method. Type 3, however, usesan ordinary epitaxial wafer in which the layers up to and including ann-InP cap layer are grown. An InGaAs filter layer at the center portionis selectively grown by utilizing the electrode structure after theformation of the pn junction.

[0073] An n-InP buffer layer 61, an n-InGaAsP absorption layer 62(λg=1.42 μm), and an n-InP cap layer 63 are grown epitaxially on ann-InP substrate 60. At the center portion of the chip, Zn is diffused toprovide a p-type region 64. Another p-type region is formed at theperiphery of the chip at the same time. This additional p-type region isa Zn-diffused shield layer. A ring-shaped p electrode 70 is provided onthe p-type region 64. At the outside of the ring-shaped p electrode 70,a passivation layer 67 is provided to protect the pn junction.

[0074] An InGaAs filter layer 75 that absorbs light having λ2 isselectively grown on the center portion of the chip's top surface thatis surrounded by the ring-shaped p electrode 70. The filter layer can beepitaxially grown by a method such as molecular-beam epitaxy (MBE),metalorganic chemical vapor deposition (MOCVD), etc. Because InGaAscannot grow on the passivation layer and the metal electrode, itselectively grows only on the InP cap layer. Since the p electrode liesoutside the periphery of the InGaAs filter layer 75, no electric fieldexists in the InGaAs.

[0075] A ring-shaped n electrode 68 and an anti-reflection layer 69 areformed on the bottom surface. Incoming light (λ1=1.3 μm) and outgoinglight (λ2=1.55 μm) enter at the back side. The light having λ1 isabsorbed by the absorption layer 62 and detected by generating opticalcurrent. The light having λ2 passes through the absorption layer upwardafter being absorbed slightly by the absorption layer. The light havingλ2 is absorbed mainly by the InGaAs filter layer 75. Even if a slightamount of the light having λ2 leaves the PD, the amount of the lightreflected somewhere at the outside further decreases. Virtually no lighthaving λ2 returns to the absorption layer 62. The light having λ2 passesthrough the absorption layer only once. Therefore, the crosstalk causedby the returning light having λ2 can be prevented effectively.

[0076] Type 4: Chip Whose Entire Top Surface is Covered by Plastic (FIG.15)

[0077] Types 1 to 3 absorb light having λ2 by the semiconductor materialInGaAs. However, the light can be absorbed also by plastic. FIG. 15shows such an embodiment. An n-InP buffer layer 61, an n-InGaAsPabsorption layer 62 (λg=1.42 μm), and an n-InP cap layer 63 are grownepitaxially on an n-InP substrate 60. Table II shows the thickness andcarrier concentration of the epitaxial layers. TABLE II Carrier Layer'sthickness concentration Layer's name (μm) (cm⁻³) InP cap layer (63) 3 n= 5 × 10¹⁵ InGaAsP absorption layer (62) 5 n = 10¹⁵ InP buffer layer(61) 4 n = 10¹⁵ InP substrate (60) 200  n = 10¹⁸

[0078] At the center portion of the chip, Zn is diffused to provide ap-type region 64. Another p-type region is formed at the periphery ofthe chip at the same time. This additional p-type region is aZn-diffused shield layer. A ring-shaped p electrode 70 is provided onthe p-type region 64. At the outside of the ring-shaped p electrode 70,a passivation layer 67 is provided to protect the pn junction. Noanti-reflection layer is provided at the inside of the p electrode 70. Aring-shaped n electrode 68 and an anti-reflection layer 69 are formed onthe bottom surface. They are produced through the wafer process. After achip is cut out from the wafer and mounted on a package, the p electrode70 is connected to a lead pin with a lead wire 77. Then, a plastic resin76 that absorbs light having λ2 is applied to the entire top surface ofthe chip. The plastic resin is required to absorb light having λ2; it isnot required to absorb the light exclusively. Therefore, the plasticresin can be black.

[0079] Incoming light (λ1=1.3 μm) and outgoing light (λ2=1.55 μm) enterat the back side. The light having λ1 is absorbed by the absorptionlayer and detected by generating optical current. The light having λ2passes through the absorption layer upward after being absorbed slightlyby the absorption layer. Then, the light leaves the cap layer and isabsorbed by the plastic resin 76. Only a slight amount of the lighthaving λ2 leaves the PD. Even if the light is reflected at the case andparts, it does not return to the absorption layer. As with Types 1 to 3,the light having λ2 passes through the absorption layer only once.Therefore, the crosstalk caused by the returning light having λ2 can beprevented effectively. Type 4 requires no extra epitaxial growth. Type 4accomplishes its purpose by applying the plastic resin after the chip ismounted on a package. As a result, Type 4 can be employed easily inpractical application.

EXAMPLES

[0080]FIG. 16 shows a surface-mounted-type PD module on which a Type 4PD is mounted. A V-shaped groove 81 is provided longitudinally on thesurface of an Si platform 80. A mirror face is provided at the end ofthe V-shaped groove. An optical fiber 82 is inserted into the V-shapedgroove 81 and fixed there. A back-illuminated-type PD 83 of the presentinvention is fixed on the V-shaped groove 81. Although it is aback-illuminated type, the PD has an opening at the top surface throughwhich light can pass. The top surface is covered by a plastic resin 85that absorbs light having λ2.

[0081] At a central office, the incoming light having λ1 emerges fromthe optical fiber 82, enters the V-shaped groove 81, is reflected at themirror face, and enters the PD 83 at the back side. The light reaches anabsorption layer 84, generates optical current, and is detected by thePD. When the stray light and scattered light produced by the intenseoutgoing light having λ2 enter the PD from the V-shaped groove, theabsorption layer scarcely detects them. After passing through theabsorption layer upward, they do not return to the absorption layer.There also exists stray light having λ2 above the PD. However, since itis absorbed by a plastic resin 85, it does not enter the absorptionlayer of the PD. Consequently, light having λ2 passes through theabsorption layer only once. Therefore, crosstalk caused by the lighthaving λ2 can be suppressed to 0.1% or less. The plastic resin can beapplied to the PD at the stage of the chip mounting, without increasingthe wafer process.

[0082]FIG. 17 shows a schematic cross-sectional view of asurface-mounted-type optical transceiver module that uses a Type 4 PD.The transceiver module is constructed on an Si platform, which is notshown in FIG. 17, in a housing 86. An optical fiber 87 is attached tothe Si platform. An LD 89 for generating outgoing light having λ2 at acentral office is mounted opposite to the end of the optical fiber. AWDM filter 88 is provided on the skew at some point in the optical fibernear its end. A back-illuminated-type PD 90 of the present invention isplaced obliquely above the WDM filter. The PD 90 is a Type 4 PD, whichhas on the top surface a plastic resin 92 that can absorb light havingλ2.

[0083] A signal carried by the outgoing light (λ2=1.55 μm) emitted fromthe LD 89 enters the optical fiber 87 to be transmitted to a subscriber.The incoming light (λ1=1.3 μm) carrying a signal from the subscriberpropagates through the optical fiber 87, is reflected by the WDM filter88, enters the PD 90 at the back side, and is absorbed by an absorptionlayer 91, generating optical current. The intense outgoing light havingλ2 generated by the LD 89 produces stray light and scattered light,which surround the PD. The light having λ2 which enters the PD at theback side passes through the absorption layer once, generating opticalcurrent slightly. However, the light having λ2 which enters the plasticresin 92 at the top surface is absorbed by the plastic resin, withoutpenetrating into the PD. Therefore, the module can minimize theinfluence of the outgoing light at a central office.

INDUSTRIAL APPLICABILITY

[0084] A central office receives light having a shorter wavelength(λ1=1.3 μm) and transmits light having a longer wavelength (λ2=1.55 μm).When the bandgap wavelength of a semiconductor is represented by λg, thelight having λ2, which is longer than λg, was thought to pass throughthe semiconductor without causing any influence. Therefore, thephenomenon that the light having λ2 causes crosstalk to the light havingλ1 was not known to a person skilled in the art. There was no conceptthat light having a longer wavelength causes crosstalk to a PD for ashorter wavelength. In other words, the present inventors first foundthe necessity of reducing the above-mentioned crosstalk.

[0085] The present inventors first found that contrary to theconventional knowledge, the absorption layer of a PD for a shorterwavelength, λ1, slightly detects a longer wavelength, λ2. The presentinvention admits that in a back-illuminated-type PD for λ1, light havingλ2 passes through the absorption layer once. The present invention,however, prevents the light from passing through the layer again afterbeing reflected. The first passage generates a crosstalk of 0.1%; thesecond passage increases it to 0.2%. The present invention prevents thisincrease and thereby enables the reduction of the crosstalk hithertounknown to a person skilled in the art.

What is claimed is:
 1. A back-illuminated-type photodiode thatconcurrently admits light having a longer wavelength, λ2, and lighthaving a shorter wavelength, λ1, and that detects only the light havinga shorter wavelength, λ1, which photodiode comprising: (a) asemiconductor substrate; (b) an absorption layer being: (b1) placed onthe semiconductor substrate; (b2) provided with a pn junction in thelayer; and (b3) made of a semiconductor material having a bandgapwavelength, λg, longer than λ1 and shorter than λ2; (c) a cap layerbeing: (c1) placed on the absorption layer; and (c2) made of a materialthat absorbs light having the wavelength λ2; (d) a first electrodebeing: (d1) placed on the bottom surface of the semiconductor substrate;and (d2) provided with an opening for admitting light; and (e) a secondelectrode being: (e1) placed on the cap layer; and (e2) provided with anopening for allowing light to leave the photodiode.
 2. Aback-illuminated-type photodiode that concurrently admits light having alonger wavelength, λ2, and light having a shorter wavelength, λ1, andthat detects only the light having a shorter wavelength, λ1, whichphotodiode comprising: (a) a semiconductor substrate; (b) an absorptionlayer being: (b1) placed on the semiconductor substrate; (b2) providedwith a pn junction in the layer; and (b3) made of a semiconductormaterial having a bandgap wavelength, λg, longer than λ1 and shorterthan λ2; (c) a cap layer placed on the absorption layer; (d) a filterlayer being: (d1) placed on the center portion of the cap layer; and(d2) made of a material that absorbs light having the wavelength λ2; (e)a first electrode being: (e1) placed on the bottom surface of thesemiconductor substrate; and (e2) provided with an opening for admittinglight; and (f) a second electrode being: (f1) placed on the filterlayer; and (f2) provided with an opening for allowing light to leave thephotodiode.
 3. A back-illuminated-type photodiode that concurrentlyadmits light having a longer wavelength, λ2, and light having a shorterwavelength, λ1, and that detects only the light having a shorterwavelength, λ1, which photodiode comprising: (a) a semiconductorsubstrate; (b) an absorption layer being: (b1) placed on thesemiconductor substrate; (b2) provided with a pn junction in the layer;and (b3) made of a semiconductor material having a bandgap wavelength,λg, longer than λ1 and shorter than λ2; (c) a cap layer placed on theabsorption layer; (d) a filter layer being: (d1) placed on the centerportion of the cap layer; and (d2) made of a material that absorbs lighthaving the wavelength λ2; (e) a first electrode being: (e1) placed onthe bottom surface of the semiconductor substrate; and (e2) providedwith an opening for admitting light; and (f) a second electrode placedat the outside of the filter layer, thereby forming an opening forallowing light to leave the photodiode.
 4. A back-illuminated-typephotodiode that concurrently admits light having a longer wavelength,λ2, and light having a shorter wavelength, λ1, and that detects only thelight having a shorter wavelength, λ1, which photodiode comprising: (a)a semiconductor substrate; (b) an absorption layer being: (b1) placed onthe semiconductor substrate; (b2) provided with a pn junction in thelayer; and (b3) made of a semiconductor material having a bandgapwavelength, λg, longer than λ1 and shorter than λ2; (c) a cap layerplaced on the absorption layer; (d) a first electrode being: (d1) placedon the bottom surface of the semiconductor substrate; and (d2) providedwith an opening for admitting light; (e) a second electrode being: (e1)placed on the cap layer; and (e2) provided with an opening for allowinglight to leave the photodiode; and (f) a plastic resin being: (f1)applied in such a manner as to form the topmost layer of the photodiode;and (f2) capable of absorbing light having the wavelength λ2.
 5. Aphotodiode as defined in any of claims 1 to 4 in which: (a) thewavelength λ1 is not less than 1.2 μm and not more than 1.38 μm; and (b)the wavelength λ2 is not less than 1.45 μm and not more than 1.65 μm. 6.A photodiode as defined in any of claims 1 to 4 in which the absorptionlayer comprises InGaAsP having a bandgap wavelength not less than 1.3 μmand not more than 1.5 μm.
 7. A photodiode as defined in claim 6 in whicha buffer layer comprising InP is provided between the substrate and theabsorption layer.
 8. A photodiode as defined in claim 6 or 7 in which aZn-diffused shield layer constituting a p-type region is provided aroundthe pn junction at the center portion of the InGaAsP absorption layer.9. A photodiode as defined in any of claims 1 to 4 in which ananti-reflection layer is provided at the opening of the first electrode.10. A photodiode as defined in claim 1 or 2 in which an anti-reflectionlayer is provided at the opening of the second electrode.